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The speed and level of chaos in Iraq is picking up fast. An apocalyptic cult came uncomfortably close to taking Najaf, one of Shi'a Islam's most holy cities, and murdering Grand Ayatollah Sistani. Sistani is the neo-cons' favorite quietist Shi'a cleric, the man who was supposed to keep Iraq's Shi'a in line while we went about nation building. And then, on Sunday, Iran's ambassador to Baghdad told the New York Times that Iran is in Iraq to stay, whether the Bush Administration likes it or not.

And that's not the worst of it. American forces still hold five members of Iran's Islamic Revolutionary Guard Corps. Arrested by American forces in Erbil on Jan. 11, the five are accused by the Administration of helping the Iraqi opposition kill Americans.

I've written here before that the IRGC has a long history of calculated violence against its enemies, particularly the United States. The Administration's accusations are plausible. But at the same time the U.S. needs to remember what a serious spoiler the IRGC can be when provoked.

In July 1982, after a Christian Lebanese militia kidnapped the Iranian charge d'affaires in Beirut, the IRGC set in motion a campaign of retaliatory kidnappings, hijackings and assassinations against the U.S. and the West. The Iranian charge was a senior IRGC officer, and the IRGC had no intention of letting his kidnapping go unanswered. The IRGC campaign lasted for more than 10 years and dragged the U.S. into Iran-contra and the arms-for-hostages deal that nearly brought down the Reagan Administration.

Some Iraqis speculate that the IRGC has already started a campaign of revenge with the killing of five American soldiers in Karbala on Jan. 20, nine days after the arrest of the IRGC members in Erbil. As the logic of the rumor goes, five American soldiers were killed for five Iranians taken; Karbala was an IRGC message to release its colleagues  or else.

The speculation that Karbala was an IRGC operation may have as much to do with Iraqis' respect for IRGC capacity for revenge as it does with the truth. Nevertheless, we should count on the IRGC gearing up for a fight. And we shouldn't underestimate its capacities. Aside from arming the opposition, the IRGC is capable of doing serious damage to our logistics lines. I called up an American contractor in Baghdad who runs convoys from Kuwait every day and asked him just how much damage. "Let me put it this way," he said. "In Basra today the currency is the Iranian toman, not the Iraqi dinar." He said his convoys now are forced to pay a 40% surcharge to Shi'a militias and Iraqi police in the south, many of whom are affiliated with IRGC.

Mindful of the spreading chaos in Iraq, President Bush has promised not to take the war into Iran. But it won't matter to the IRGC. There is nothing the IRGC likes better than to fight a proxy war in another country.

Robert Baer, a former CIA field officer assigned to the Middle East, is the author of See No Evil and, most recently, the novel Blow the House Down.